Electrolyzer

ABSTRACT

Provided are an electrolyzer having excellent durability against reverse current. The electrolyzer 300 includes an anode 314, an anode chamber 310 housing the anode 314, a cathode 330, a cathode chamber 320 housing the cathode 330, and a diaphragm that separates the anode chamber 310 and the cathode chamber 320, wherein a reverse current absorption body 334 formed of a sintered compact containing nickel is disposed in at least one of an inside of the cathode chamber 320 and an inside of the anode chamber 310, and the reverse current absorption body 334 is not directly coupled to the cathode 330 and the anode 314 but is electrically connected to at least one of the cathode 330 and the anode 314.

TECHNICAL FIELD

The present invention relates to an electrolyzer, and in particular,relates to an electrolyzer for electrolysis that is applicable to analkali metal aqueous solution.

BACKGROUND ART

Known types of electrolysis include alkali chloride electrolysis such asbrine electrolysis, and alkali metal aqueous solution electrolysis suchas alkaline water electrolysis and alkali sulfate electrolysis. In analkali metal aqueous solution electrolytic apparatus, the electrolyzerhouses a plurality of internal electrolytic cells. Each electrolyticcell has a cathode chamber that houses a cathode, an anode chamber thathouses an anode, and a partition wall that separates the cathode chamberand the anode chamber. The inside of the electrolyzer is arranged sothat the cathode chamber and anode chamber of adjacent electrolyticcells oppose one another, and a diaphragm is disposed between theelectrolytic cells. For example, in a brine electrolytic apparatus, anion exchange membrane method that uses an electrolyzer containing an ionexchange membrane as the diaphragm is often used (Patent Document 1).

In electrolysis using the type of electrolyzer described above, ifoperation of the electrolyzer is stopped due to trouble or the like,then a reverse current (a current in the reverse direction to theelectrolytic current) flows through the electrolyzer. Particularly inthe case of bipolar electrolyzers, which are the most common form ofelectrolyzers for brine electrolysis, the size of the reverse currentincreases in proportion to the square of the number of electrolyticcells. In recent years, there is a trend toward larger electrolyzers,resulting in an associated increase in the number of electrolytic cells.As a result, the size of the reverse current that flows when theelectrolysis is stopped has also increased.

As a result of this reverse current flow, cathode degradation occurs inwhich the cathode catalyst (noble metal material) is eluted as a resultof oxidation. In recent years, rather than platinum (Pt) or rhodium(Rh), the less expensive ruthenium (Ru) has become more widely used asthe cathode catalyst material. However, because Ru is easily eluted as aresult of reverse current, a more effective countermeasure forpreventing oxidation caused by reverse current is required.

In order to prevent cathode degradation caused by reverse current, onemeasure that has been taken involves applying a very weak currentthrough the electrolyzer during shutdowns of the electrolyzer, therebymaintaining the cathode potential at the hydrogen generation potential.However, there is a risk that the generated hydrogen may pass throughthe diaphragm, diffuse into the anode side, and mix with the oxygen gasgenerated at the anode side to form an explosive gas, and this risk mustbe avoided. Accordingly, initial capital costs and operating costs tendto increase as a result of the complexity of the operating procedure andthe requirement for additional equipment.

Another countermeasure that has been proposed for suppressing cathodedegradation caused by reverse current flow during operational shutdownsinvolves placing a material containing a substance that preferentiallyabsorbs the reverse current inside the cathode chamber.

Patent Document 1 proposes providing a reverse current absorption layer,which is connected electrically to the cathode, inside the cathodechamber. The reverse current absorption layer in Patent Document 1contains a material having a lower redox potential than the cathodematerial. Because the reverse current is consumed by an oxidationreaction of the reverse current absorption layer rather than thecathode, oxidative degradation of the cathode due to the reverse currentis suppressed. The reverse current absorption layer of Patent Document 1is formed by a deposition technique such as thermal spraying onto asubstrate such as a current collector, metal elastic body or partitionwall inside the electrolytic cell. Alternatively, a reverse currentabsorption body composed of a reverse current absorption layer formed ona separate independent substrate may be attached to an electrolytic cellcomponent such as a current collector or metal elastic body.

Patent Document 2 proposes a cathode structure including an activecathode, a cathode current collector and an elastic cushion material,wherein at least a surface layer of the cathode current collector iscomposed of an active material that can consume a greater oxidationcurrent per unit of surface area than the active cathode. Specificexamples of this type of active material include Raney nickel, Raneynickel alloys, activated carbon-nickel composite plating, and compositeplating of hydrogen storage alloy particles. When the electrolyzer isstopped and a reverse current flows, this active material on the cathodecurrent collector preferentially consumes the oxidation current,enabling oxidation of the active cathode that accompanies anodicpolarization to be suppressed to a minimum.

CITATION LIST Patent Documents

-   Patent Document 1: International Patent Application No. 2013/141211-   Patent Document 2: International Patent Application No. 2012/032793

SUMMARY OF INVENTION Technical Problem

In Patent Document 1 and Patent Document 2, a substrate is required forproviding the thin film-like reverse current absorption layer. In thosecases where the substrate is a structural component of the electrolyticcell, because the substrate shape is large and complex, formation of thereverse current absorption layer is not easy. Once the reverse currentabsorption layer has been consumed, the entire structural component mustbe replaced, meaning maintenance is complicated and the costs are high.

Further, in the case of the reverse current absorption body disclosed inPatent Document 1, because a separate substrate must be provided, thiscauses an increase in the material costs.

Moreover, in Patent Document 1 and Patent Document 2, because a thinfilm-like reverse current absorption layer is used, the amount of thereverse current absorption material is small. As a result, the reversecurrent absorption performance is low, and when a large reverse currentoccurs due to the size increase of the electrolyzer mentioned above, thepreventive effect on cathode oxidation may be insufficient. In order toenable the absorption of more reverse current, the surface area of thereverse current absorption layer and the amount used of the reversecurrent absorption material must be increased.

However, when the reverse current absorption layer is formed using astructural component of the cell as a substrate, because the surfacearea of the substrate is limited, increasing the amount of the reversecurrent absorption material is difficult.

On the other hand, when the reverse current absorption body of PatentDocument 1 is used, the substrate surface area must be increased toincrease the amount of the reverse current absorption material. However,this results in a significant increase in material costs. Moreover, inorder to ensure satisfactory reverse current absorption, the regionoccupied by the reverse current absorption body inside the cathodechamber increases, and there is a possibility that the electrolysis maybe affected. Further, from the viewpoint of the capacity of the cathodechamber, there is a limit to the size of reverse current absorption bodythat can be installed, which is still not entirely satisfactory forapplication within an electrolyzer.

The present invention has been developed in light of the abovecircumstances, and has an object of providing an electrolyzer that hasexcellent durability against reverse current.

Solution to Problem

An aspect of the present invention is an electrolyzer that includes ananode, an anode chamber housing the anode, a cathode, a cathode chamberhousing the cathode, and a diaphragm that separates the anode chamberand the cathode chamber, wherein a reverse current absorption bodyformed of a sintered compact containing nickel is disposed in at leastone of an inside of the cathode chamber and an inside of the anodechamber, and the reverse current absorption body is not directly coupledto the cathode and the anode but is electrically connected to at leastone of the cathode and the anode.

In the aspect of the present invention, the electrolyzer furtherincludes a cathode current collector disposed in the cathode chamber andopposed to the cathode, wherein the reverse current absorption body iscoupled to the cathode current collector.

In the aspect of the present invention, the electrolyzer furtherincludes an anode current collector disposed in the anode chamber andopposed to the anode, wherein the reverse current absorption body iscoupled to the anode current collector.

Further, in the aspect of the present invention, the reverse currentabsorption body is preferably attached to at least one of a frame bodydefining the cathode chamber and a first support member disposed in thecathode chamber and supporting the cathode.

Further in the aspect of the present invention, the reverse currentabsorption body is preferably attached to at least one of a frame bodydefining the anode chamber and a second support member disposed in theanode chamber and supporting the anode.

Moreover, in the aspect of the present invention, the content of nickelin the sintered compact containing nickel is preferably 45 to 90 mass %.

Moreover, in the aspect of the present invention, a density of thereverse current absorption body is preferably 2.00 to 6.51 g/cm³.

Effects of Invention

According to the present invention, by using a reverse currentabsorption body formed from a sintered compact containing nickel, a highdischarge capacity can be obtained. In other words, the reverse currentabsorption body according to the present invention is able to adequatelyimprove the reverse current resistance of the electrode for electrolysiseven in small volume. Further, because a substrate is not required, thematerial costs can be reduced. Moreover, compared with a thin film-likereverse current absorption layer, the reverse current absorption bodyaccording to the present invention can be produced extremely easily.

The reverse current absorption body of the present invention can beeasily attached to and removed from the substrate, and can therefore becoupled to the substrate in a location appropriate for the electrolyzerspecifications. Further, the reverse current absorption body also offersthe advantage of being easy to replace during maintenance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front schematic view describing one example of anelectrolytic cell.

FIG. 2 is a diagram describing an electrolytic cell of a firstembodiment, and illustrates a cross-section along A-A′ in FIG. 1.

FIG. 3 is a diagram describing an electrolytic cell of another exampleof the first reference embodiment, and illustrates a cross-section alongA-A′ in FIG. 1.

FIG. 4 is a diagram describing an electrolytic cell of a secondreference embodiment, and illustrates a cross-section along A-A′ in FIG.1.

FIG. 5 is a diagram describing an electrolytic cell of a firstembodiment, and illustrates a cross-section along A-A′ in FIG. 1.

FIG. 6 is a diagram describing an electrolytic cell of a secondembodiment, and illustrates a cross-section along A-A′ in FIG. 1.

FIG. 7 is a diagram describing an electrolytic cell of a thirdembodiment, and illustrates a cross-section along A-A′ in FIG. 1.

FIG. 8 is a diagram describing an electrolytic cell of the thirdembodiment, and illustrates a cross-section along B-B′ in FIG. 1.

FIG. 9 is a diagram describing an electrolytic cell of a fourthembodiment, and illustrates a cross-section along A-A′ in FIG. 1.

FIG. 10 is a graph illustrating the change in the hydrogen overvoltageduring reverse current application cycles.

FIG. 11 is a diagram illustrating the relationship between the dischargecapacity and the sintered compact density in a case where metallicnickel particles having an average particle size of 4 μm and Raneynickel are used.

FIG. 12 is a diagram illustrating the relationship between the dischargecapacity and the sintered compact density in cases where metallic nickelparticles having average particle sizes of 4 μm and 50 μm and Raneynickel are used.

FIG. 13 is a diagram illustrating the relationship between the dischargecapacity and the sintered compact density in cases where metallic nickelparticles having average particle sizes of 4 μm, 50 μm and other averageparticle sizes, and Raney nickel are used.

DESCRIPTION OF THE EMBODIMENTS

Embodiments and reference embodiments of the present invention aredescribed below with reference to the drawings. However, thedescriptions below and the drawings are merely examples, and the presentinvention should not be interpreted as being limited to these examples.Embodiments having variations including all manner of designmodifications are also included within the scope of the presentinvention, provided they exhibit the effects described in each of theembodiments and reference embodiments.

FIG. 1 is a front schematic view describing one example of anelectrolytic cell used in an alkali metal aqueous solution electrolyticapparatus. “Alkali metal aqueous solution electrolysis” is a genericterm for electrolysis that uses an aqueous solution containing alkalimetal ions as an electrolyte. This includes, for example, electrolysisof an electrolyte containing an alkali metal chloride (alkali chlorideelectrolysis) such as brine electrolysis, an alkaline water electrolyticapparatus, and electrolysis of an electrolyte containing an alkali metalsulfate (alkali sulfate electrolysis).

The external appearance (front view) of an electrolytic cell 10 iscommon to each of the embodiments and reference embodiments describedbelow. The electrolytic cell 10 has a gasket 14 composed of arectangular-shaped frame. An electrode (anode or cathode) 12 ispositioned within the open portion of the gasket 14. Although not shownin FIG. 1, a supply nozzle for supplying an electrolyte into theinterior of the electrolytic cell 10, and a discharge nozzle fordischarging the electrolyte inside the electrolytic cell 10 to anexternal location are attached to the electrolytic cell 10.

When a plurality of the electrolytic cells 10 are housed inside anelectrolyzer, the cells are arranged so that the cathode and anode ofadjacent electrolytic cells oppose one another, and a diaphragm isprovided between the electrolytic cells.

First Reference Embodiment

A first reference embodiment of the present invention is described usinga bipolar brine electrolytic apparatus. FIG. 2 illustrates anelectrolytic cell used in the brine electrolytic apparatus, andillustrates a cross-sectional view along A-A′ in FIG. 1 (a horizontalcross-sectional view).

In an electrolytic cell 100 in the first reference embodiment, an anodechamber 110 and a cathode chamber 120 are demarcated inside theelectrolytic cell 100 by a frame body. In the electrolytic cell 100 ofFIG. 1, this frame body is composed of a frame-like frame 102 and apartition wall 104 that partitions the interior of the frame 102, withthe frame 102 and the partition wall 104 forming the anode chamber 110and the cathode chamber 120 inside the electrolytic cell 100. The frame102 has gasket-bearing surfaces 102A that protrude outward from theframe 102. The gasket-bearing surfaces 102A and the gasket 106 arejoined together by fastening means (not shown in the drawing).

An anode 114 is installed in the open portion within the frame 102 ofthe anode chamber 110. A plurality of support members (ribs) 108 areattached to the partition wall 104, and the anode 114 is supported bythese support members 108. The anode 114 is a metal electrode having acatalyst layer formed on the surface of a conductive substrate. Theconductive substrate of the anode 114 is formed from titanium, and is amember having a plurality of through holes, such as an expanded mesh,punched metal, or wire gauze or the like. Known metals such asruthenium, platinum, iridium and titanium, or oxides of these metals,can be used for the catalyst of the anode 114. A buffer plate 116 isinstalled inside the anode chamber 110. The buffer plate 116 promotesliquid circulation inside the anode chamber 110, and has the role offacilitating a uniform concentration distribution for the electrolyteinside the anode chamber.

A cathode structure 122 is installed inside the cathode chamber 120. Thecathode structure 122 includes a cathode 130, a cathode currentcollector 128 and an elastic body 126. As illustrated in FIG. 2, thecathode 130 is positioned within the open portion of the frame 102. Thecathode current collector 128 is positioned opposing the cathode 130,and is located closer to the partition wall 104 than the cathode 130.The elastic body 126 is disposed between the cathode 130 and the cathodecurrent collector 128. The cathode 130 and the elastic body 126 contacteach another, and the elastic body 126 and the cathode current collector128 contact each other. As a result, the cathode 130 and the cathodecurrent collector 128 are connected electrically via the elastic body126.

A plurality of support members 108 are also attached to the partitionwall 104 inside the cathode chamber 120, and the cathode currentcollector 128 is supported by these support members 108. As a result,the cathode 130 is supported by the support members 108, via the cathodecurrent collector 128 and the elastic body 126. In the case of anelectrolyzer having a structure that lacks the partition wall, thesupport members may be attached to the frame body (frame) or the like.FIG. 2 illustrates a structure having a cathode structure, but in a casewhere the cathode current collector and elastic body are absent, thecathode may be supported directly by the support members. The supportmembers may be integrated with the cathode current collector and theelastic body or may be integrated with the cathode.

The cathode current collector 128 is a member composed of nickel or anickel alloy or the like. There are no particular limitations on theform of the cathode current collector 128, which may be mesh-like orplate-like.

The elastic body 126 has the roles of supplying electricity to thecathode 130, as well as pressing the cathode 130 against the diaphragmwhen a plurality of electrolytic cells 100 are arranged in alignment,thereby closing the distance between the cathode 130 and the anode 114of the adjacent electrolytic cell 100. By reducing the distance betweenthe anode 114 and the cathode, the voltage can be reduced and the powerconsumption can be lowered when the plurality of electrolytic cells 100are aligned. Examples of materials that can be used as the elastic body126 include non-rigid members such as woven fabric, nonwoven fabric ormeshes formed from fine metal wires, as well as plate-like springs,spiral springs, and coiled cushions and the like. The elastic body 126is formed from a metal material such as nickel, a nickel alloy, orsilver or the like, which has a low specific resistance and exhibitsexcellent corrosion resistance relative to alkali.

The cathode 130 in the present reference embodiment is composed of aconductive substrate 132 and a reverse current absorption body 134. Theend portions of the conductive substrate 132 are bent toward theinterior of the cathode chamber 120.

A catalyst layer is formed on the surface of the conductive substrate132. The catalyst layer contains a noble metal such as platinum,ruthenium, iridium, rhodium, palladium or silver, or an oxide of one ofthese noble metals. The catalyst layer may also contain an alloy which,in addition to one of the above noble metals, includes an element suchas nickel, cerium, lanthanum, praseodymium, rhodium or palladium, or maycontain an oxide of such an alloy. Specific examples of the catalystlayer include Ru—La—Pt-based, Ru—Ce-based, Pt—Ni-based, Ru—Pr-based,Pt—Ru—Pr-based, Pt—Pd—Pr-based, Pt—Rh—Pd—Pr-based and Pt—Ce-basedmaterials. The substrate of the conductive substrate 132 is formed fromnickel or a nickel alloy, and is a member having a plurality of throughholes, such as an expanded mesh, punched metal, or wire gauze or thelike.

The reverse current absorption body 134 is coupled so as to directlycontact the conductive substrate 132. A plurality of reverse currentabsorption bodies 134 may be provided for a single conductive substrate132. The number of reverse current absorption bodies 134 installed isdetermined in accordance with the requirements specification of theelectrolyzer.

There are no particular limitations on the location in which the reversecurrent absorption body 134 is installed. In the example in FIG. 2, thereverse current absorption bodies 134 are disposed at the end portionsof the conductive substrate 132. In the example in FIG. 2, the endregions of the conductive substrate 132 are bent, and the reversecurrent absorption bodies 134 are installed within these bent portionsthat face the gasket 106.

FIG. 3 illustrates an electrolytic cell 140 of another example of thepresent reference embodiment, wherein the reverse current absorptionbody 134 is disposed in a region other than the end portions of theconductive substrate 132, such as the central portion of the conductivesubstrate 132. In this case, the reverse current absorption body 134 maybe sandwiched between the cathode 130 and the elastic body 126. Althoughdependent on the size and number of reverse current absorption bodies134, installing the reverse current absorption bodies 134 at the endportions of the conductive substrate 132, namely in regions close to theedges of the conductive substrate 132, reduces inhibition of thecirculation of the electrolyte and lowers the possibility of any adverseeffects on the state of the electrolysis, and is consequently preferred.

The reverse current absorption body 134 is coupled to the conductivesubstrate 132 to form an integrated body. In the present referenceembodiment, the reverse current absorption body 134 is coupled to theconductive substrate 132 in a detachable manner. The reverse currentabsorption body 134 is connected electrically to the conductivesubstrate 132.

The reverse current absorption body 134 may be welded to the conductivesubstrate 132. In this case, the two members are preferably partiallyjoined by spot welding or the like. The reverse current absorption body134 may also be secured to the conductive substrate 132 by a wirematerial such as metallic wire. The reverse current absorption body 134may also be coupled to the conductive substrate 132 by sandwiching thereverse current absorption body 134 between the conductive substrate 132and the elastic body 126. The reverse current absorption body 134 mayalso be coupled to the conductive substrate 132 by curving the edgeportions of the conductive substrate 132, and wrapping the reversecurrent absorption body 134 in the curved portions.

Alternatively, the reverse current absorption body 134 may be coatedwith a wire gauze such as a woven mesh that is coupled to the conductivesubstrate 132 by one of the methods described above. In this case, agood electrical connection is maintained between the conductivesubstrate 132 and the reverse current absorption body 134.

The reverse current absorption body 134 is formed from a sinteredcompact containing nickel, which is a less-noble metal than the catalystlayer of the cathode. In other words, the reverse current absorptionbody 134 of the present reference embodiment is composed only of asintered compact, and unlike Patent Document 1 and Patent Document 2, asubstrate used for supporting a layer having reverse current absorptionperformance is not required.

Reverse current is an electric current which, when electrolysis isstopped, flows in the reverse direction to the current duringelectrolysis. When a reverse current flows, the potential increaseswhile a variety of oxidation reactions proceed at the cathode, withthose reactions of lower redox potential occurring first. Theequilibrium potential of the oxidation reaction of the cathode catalystmaterial is more noble than the equilibrium potential of the oxidationreaction of nickel. For example, when ruthenium is used as the catalystmaterial, reverse current causes oxidation reactions to proceed in thefollowing order.

H_(ad) (atomic hydrogen adsorbed to the electrode surface)+OH⁻→H₂O+e⁻  (1)

Ni+2OH⁻→Ni(OH)₂+2e ⁻  (2)

Ru+4OH⁻→RuO₂+2H₂O+4e ⁻  (3)

RuO₂+4OH⁻→RuO₄ ²⁻+2H₂O+2e ⁻  (4)

During the period while a single oxidation reaction proceeds, thepotential is maintained at a constant value. Accordingly, followingcompletion of the oxidation reaction (1) of adsorbed hydrogen generatedby electrolysis and the oxidation reaction (2) of the nickel in thereverse current absorption body, the oxidation reaction (3) of rutheniumand then an elution reaction (4) of the catalyst (ruthenium) occur. Inthe present description, the electric capacity of the period up untilcompletion of the reactions (1) and (2) is defined as the dischargecapacity.

As is evident from the above description, the larger the amount ofatomic hydrogen adsorbed to the electrode surface and the larger theamount of nickel, the greater the amount of reverse current consumed bythe reactions (1) and (2). This state is called “reverse currentabsorption”.

The reverse current absorption body 134 is a bulk material. Comparedwith the reverse current absorption bodies of Patent Document 1 andPatent Document 2 which have a thin-film reverse current absorptionlayer formed on a substrate, a reverse current absorption body 134 of asimilar size has a much larger amount of the component (mainly Ni) thatcontributes to reverse current absorption. Accordingly, the reversecurrent absorption body 134 of the present reference embodiment hassuperior reverse current absorption performance. Although ruthenium isrelatively inexpensive, it dissolves extremely easily in alkali uponanodic polarization. As a result, it can be said to be a material thatis susceptible to reverse current. The reverse current absorption body134 of the present reference embodiment is able to satisfactorilysuppress cathode degradation even when ruthenium is used as a catalyst.

There are no particular limitations on the shape of the reverse currentabsorption body 134. The reverse current absorption body 134 may have aprismatic, flat plate-like, or rod-like shape, and may have grooves orthe like formed therein to facilitate coupling to the conductivesubstrate 132 or as a result of the production specifications. However,when the reverse current absorption body 134 is formed with a thinplate-like shape, the installation area increases. This can causevarious problems as the number of reverse current absorption bodies thatcan be installed is more limited, the reverse current absorptionperformance for the overall electrolyzer decreases, and circulation ofthe electrolyte is more likely to be inhibited by the reverse currentabsorption body. Further, a thin plate-like reverse current absorptionbody may also suffer from strength problems. For bodies having the sameprojected surface area, a thicker reverse current absorption body has agreater nickel content, and therefore has a larger discharge capacity.However, in the case of a thick reverse current absorption body, theelectrolyte is unable to penetrate into the interior of the body,meaning the internal Ni is unable to contribute to the reverse currentabsorption performance. Further, in the case of a thick reverse currentabsorption body, the installation space required inside the cathodechamber increases. For these reasons, there is an upper limit for thethickness of the reverse current absorption body. The optimal value forthe thickness of the reverse current absorption body 134 differsdepending on the size of the electrolyzer, and the size and shape of thereverse current absorption body. The thickness of the reverse currentabsorption body 134 is determined with due consideration of the factorsdescribed above.

Examples of the raw material powder for the reverse current absorptionbody 134 (nickel-containing sintered compact) include metallic nickelparticles, Raney nickel alloy particles, and mixtures of the two. A“Raney nickel alloy” contains an alkali-soluble metal element (Al, Si,Sn, Zn) and nickel. Examples of Raney nickel alloys that may be used inthe present reference embodiment include binary alloys such as Ni—Al andNi—Si, as well as multicomponent alloys containing one or more addedmetal elements such as Ru, Co, Ti, Mn, Cu, Fe and Mo besides the nickeland the alkali-soluble metal element.

An aluminum stearate such as aluminum tristearate may be added to theraw material powder as an additive.

The size of the metallic nickel particles is one factor that affects theperformance (discharge capacity) of the reverse current absorption body134. The size of the metallic nickel particles, reported as an averageparticle size measured using a FSSS (Fischer sub-sieve sizer), ispreferably at least 0.13 μm but not more than 50 μm, more preferably atleast 0.2 μm but not more than 10 μm, and most preferably at least 2 μmbut not more than 5 μm. When the metallic nickel particles are small,the specific surface area of the sintered compact increases and thedischarge capacity increases, which is beneficial. However, if theparticles are too small, then the sintered compact becomes excessivelydense, making it more difficult for the electrolyte to penetrate intothe interior of the sintered compact. As a result, the portion of thecompact that contributes to the discharge reaction decreases and thedischarge capacity decreases. On the other hand, if the metallic nickelparticles are larger than the particle size range described above, thenthe specific surface area decreases, causing a reduction in thedischarge capacity.

Further, the nickel content in the reverse current absorption body 134is preferably from 45 to 90% by mass. Provided the nickel content fallswithin this range, the shape of the reverse current absorption body 134can be maintained even after the immersion step, and a high dischargecapacity exceeding 1.5 mF/g (10 F/m²) can be obtained. From theviewpoint of durability, this nickel content is more preferably from 53to 90% by mass. Moreover, if consideration is given to the reversecurrent absorption performance, then the nickel content is even morepreferably from 53 to 87.5% by mass, and is most preferably from 53 to77.5% by mass.

Further, the density of the reverse current absorption body 134 ispreferably from 2.00 to 6.51 g/cm³. Provided the density falls withinthis range, the shape of the reverse current absorption body 134 can bemaintained even after the immersion step, and a high discharge capacityexceeding 1.5 mF/g (10 F/m²) can be obtained. From the viewpoint ofdurability, this density is more preferably from 2.30 to 6.51 g/cm³.Moreover, if consideration is given to the reverse current absorptionperformance, then the density is even more preferably from 2.30 to 5.95g/cm³, and is most preferably from 2.30 to 5.10 g/cm³.

In the present reference embodiment, there are no particular limitationson the shape of the Raney nickel alloy particles and the metallic nickelparticles, and particles of all manner of shapes including spherical,spheroidal, polyhedral and irregular shapes can be used.

The reverse current absorption body 134 is formed by the steps describedbelow.

The raw material powder described above is molded into a prescribedshape in accordance with the required specifications. Because themolding pressure is a parameter that affects the sinterability and theporosity of the sintered compact, the external appearance (including thepresence or absence of cracks) and the discharge capacity and the likeof the sintered compact can be altered by altering the molding pressure.Particularly in those cases where the raw material powder is composed ofonly metallic nickel particles, or composed of a mixture in which theproportion of metallic nickel particles is large, cracking may occurduring sintering depending on the molding pressure. Similarly, in thosecases where the raw material powder is composed of Raney nickel alloyparticles, cracking may occur during sintering depending on the moldingpressure. Following molding of the raw material powder, the molded bodyis sintered (in the sintered compact formation step). Examples ofmolding methods that may be used include conventional methods such aspress molding and cold isostatic pressing methods, as well as metalpowder injection molding and extrusion molding methods.

In the present embodiment, rather than using the formation stepsdescribed above, a conventional method such as a hot press method, hotisostatic press method or spark plasma sintering method may be used toform the molded body while sintering is performed.

The sintering conditions (such as the sintering temperature andsintering time) may be set appropriately in accordance with thesinterability and the desired external appearance for the sinteredcompact and the like.

The obtained sintered compact is coupled to the conductive substrate 132using the types of means described above (the coupling step). In thepresent embodiment, the sintered compact may be immersed in an alkalisolution following sintering and then coupled to the conductivesubstrate 132 (production step A described below), or followingsintering, the sintered compact may be coupled, as is, to the conductivesubstrate 132 (production step B described below).

(Production Step A)

In the production step A, following sintering, the sintered compact isimmersed in an aqueous solution containing an alkali metal hydroxide(such as NaOH or KOH) (pre-coupling immersion step). This pre-couplingimmersion step elutes any alkali-soluble components (alkali-solublemetal elements) near the surface of the sintered compact. The immersionconditions may be set appropriately in accordance with the size of thereverse current absorption body, the elution rate of the alkali-solublecomponents, and the desired immersion time and the like. For example,the elution conditions may include an immersion temperature of 25° C.(room temperature) to 100° C., an alkali (NaOH) concentration of 1 to 40wt %, and an immersion time of 1 to 24 hours.

The sintered compact that has been subjected to the pre-couplingimmersion step is coupled as the reverse current absorption body 134 tothe conductive substrate 132 of the cathode 130. The method used for thecoupling is as described above.

This cathode 130 forms the cathode structure 122 in combination with thecathode current collector 128 and the elastic body 126. This cathodestructure 122 is incorporated into the electrolytic cell 100, and theelectrolytic cell 100 is housed inside the electrolyzer.

Because the reverse current absorption body of the present referenceembodiment is a bulk material, alkali-soluble components remain withinthe interior of the reverse current absorption body even after theelution step. When this reverse current absorption body is incorporatedinto an electrolytic cell and electrolysis is performed, thealkali-soluble components are eluted into the electrolyte over longperiods of operation. In the production step A, because the reversecurrent absorption body is incorporated into the electrolyzer after thealkali-soluble components at the surface of the reverse currentabsorption body have already been removed, the amount of impuritiesincorporated within the product can be reduced. This production step iseffective in those cases where a high-purity product (sodium hydroxide)is required, and in those cases where the amount of Raney nickel alloyparticles in the raw material powder is large.

(Production Step B)

In the production step B, the sintered compact is coupled to theconductive substrate 132 without performing the pre-coupling immersionstep described above in the production step A. The coupling method is asdescribed above. The conductive substrate 132 with the attached sinteredcompact is combined with the cathode current collector 128 and theelastic body 126 to form the cathode structure 122. This cathodestructure 122 is incorporated into the electrolytic cell 100, and thenhoused inside the electrolyzer.

In the case of two adjacent electrolytic cells, the anode chamber 110 ofone electrolytic cell is disposed opposing the cathode chamber 120 ofthe other electrolytic cell. A diaphragm (for example, an ion exchangemembrane) is disposed between the two adjacent electrolytic cells. Inother words, the anode chamber 110 and the cathode chamber 120 of thetwo adjacent electrolytic cells are separated by the diaphragm.

In the case of a brine electrolytic apparatus, a sodium hydroxideaqueous solution is supplied to the cathode chamber 120 as anelectrolyte, and an electrolyte containing sodium chloride is suppliedto the anode chamber 110. Electrolysis is started after the cathode 130and the anode 114 have been immersed in these respective electrolytes.

Inside the electrolyzer, the sintered compact (the reverse currentabsorption body 134) is immersed in an electrolyte containing an alkalimetal hydroxide (post-coupling immersion step). The alkali-solublecomponents in the sintered compact (the reverse current absorption body134) (namely, the alkali-soluble components in the Raney nickel alloy)are eluted into the sodium hydroxide aqueous solution of the electrolyte(the electrolyte containing an alkali metal hydroxide). Hydrogen isgenerated as a result of this elution reaction. The eluted portionsbecome voids. As the electrolysis continues, alkali-soluble metalcontinues to be eluted from the sintered compact into the electrolyte.

Hydrogen is generated as a result of this elution reaction of thealkali-soluble components. This hydrogen adsorbs to the surface of thesintered compact, and as a result of the reaction shown above in thereaction formula (1), even the sintered compact itself immediatelyfollowing sintering has a reverse current absorption performance.Moreover, because voids are formed as a result of the immersion in thealkaline electrolyte, the electrolyte is able to penetrate more easilyinto the interior of the reverse current absorption body 134, yieldingan improvement in the reverse current absorption performance (dischargecapacity).

The amount of alkali-soluble components eluted from the sintered compactis greatest immediately after immersion, and gradually decreases as timepasses. In the case of brine electrolysis, the sodium hydroxide producedin the cathode chamber is collected as a product, and therefore theeluted alkali-soluble components become impurities within the product.The production step B can be employed in those cases where the reversecurrent absorption body 134 is formed solely from metallic nickelparticles, or in cases where a product of lower purity is permissible.

The above reference embodiment was described using a bipolar brineelectrolytic apparatus as an example, but the same effects can beobtained with a monopolar brine electrolytic apparatus. Furthermore, thestructure of the present reference embodiment can also be applied to analkali sulfate electrolytic apparatus.

Second Reference Embodiment

A second reference embodiment of the present invention is describedusing an alkaline water electrolytic apparatus. FIG. 4 illustrates anelectrolytic cell used in the alkaline water electrolytic apparatus, andillustrates a cross-sectional view along A-A′ in FIG. 1 (a horizontalcross-sectional view).

In a similar manner to the first reference embodiment, an anode chamber210 and a cathode chamber 220 are demarcated inside the electrolyticcell 200 by a frame body. The inside of the electrolytic cell 200 inFIG. 4 is divided into the anode chamber 210 and the cathode chamber 220by a frame-like frame 202 of the frame body and a partition wall 204.The frame 202 and a gasket 206 are joined together by fastening means(not shown in the drawing).

In the second reference embodiment, the electrode structures on theanode side and the cathode side are substantially the same. In otherwords, inside the cathode chamber 220 is installed a cathode structure222 containing a cathode 230, a cathode current collector 228 and acathode-side elastic body 226. Inside the anode chamber 210 is installedan anode structure 212 containing an anode 214, an anode currentcollector 218 and an anode-side elastic body 216. A plurality of supportmembers (ribs) 208 are attached to both the anode side and the cathodeside of the partition wall 204, with the anode structure 212 and thecathode structure 222 supported by these support members 208.Accordingly, the cathode 230 is supported by the support members 208 viathe cathode current collector 228 and the elastic body 226. Further, theanode 214 is supported by the support members 208 via the anode currentcollector 218 and the elastic body 216. In the case of an electrolyzerhaving a structure that lacks the partition wall, the support membersmay be attached to the frame body (frame) or the like. FIG. 4illustrates a structure having a cathode structure and an anodestructure, but in a case where the cathode current collector, the anodecurrent collector and the elastic bodies are absent, the cathode and theanode may be supported directly by the support members. The supportmembers may be integrated with the cathode current collector, the anodecurrent collector and the elastic bodies, or may be integrated with thecathode and the anode respectively. The anode 214 and the cathode 230are each disposed in the open portion of the frame 202.

The anode 214 is a metal electrode having a catalyst layer formed on thesurface of a conductive substrate. The conductive substrate of the anode214 is formed from nickel or a nickel alloy, and is a member having aplurality of through holes, such as an expanded mesh, punched metal, orwire gauze or the like. Conventional catalysts can be used for the anode214, including noble metals such as platinum and iridium, oxides ofthese metals, Raney nickel alloys, porous nickel, andnickel-cobalt-based oxides (composite oxides of nickel and cobalt, andoxides obtained by doping these types of composite oxides with manganeseor rare earth elements).

For the anode current collector 218 and the anode-side elastic body 216,the same materials as those described for the cathode current collectorand the elastic body of the first reference embodiment can be used. Thesupport members 208 may be integrated with the anode current collector218 and the elastic body 216.

The cathode structure 222 (the cathode 230, the cathode currentcollector 228, and the elastic body 226) is the same as that describedfor the first reference embodiment. As was the case in the firstreference embodiment, the support members 208 may be integrated with thecathode current collector 228 and the elastic body 226 in the secondreference embodiment.

In the second reference embodiment, a reverse current absorption body234 similar to that described in the first reference embodiment isinstalled on at least one of the anode side and the cathode side. Thereverse current absorption body 234 is coupled to the anode 214 or theconductive substrate 232 of the cathode 230. The reverse currentabsorption body 234 is coupled to the conductive substrate in adetachable manner. As a result of this coupling, a reverse currentabsorption body 234 is connected electrically to each of the electrodes(the anode 214 and the cathode 230). In a similar manner to the firstreference embodiment, the reverse current absorption bodies 234 may becoupled to the end portions of the conductive substrate, or may becoupled in a region other than the end portions, such as the centralportion of the conductive substrate. The coupling is achieved usingsimilar means to those described in the first reference embodiment suchas securing by wire, welding, or sandwiching between the conductivesubstrate and the elastic body 216 or 226. Further, the reverse currentabsorption body 234 may be coated with a wire gauze such as a woven meshbefore being coupled to the conductive substrate.

The method used for producing the reverse current absorption body 234 issimilar to that described in the first reference embodiment. In the caseof an alkaline water electrolytic apparatus, an alkaline electrolyte (anelectrolytic water containing an alkali metal hydroxide) is supplied toboth the anode chamber 210 and the cathode chamber 220, and electrolysisis performed. Accordingly, in the case of the production step B, thereverse current absorption bodies 234 are immersed in the electrolytesand undergo elution of the alkali-soluble components inside both theanode chamber 210 and the cathode chamber 220.

Further, elution of alkali-soluble components from the reverse currentabsorption bodies 234 into the electrolyte occurs in both the anodechamber 210 and the cathode chamber 220.

When a reverse current occurs, at the cathode side, the reverse currentis absorbed by the reverse current absorption body 234 in accordancewith the reactions (1) and (2), thereby suppressing elution of thecathode catalyst.

On the other hand, at the anode side, a reduction reaction (5) of theoxygen generated by electrolysis, and reduction reactions (6) and (7) ofthe nickel peroxide and nickel oxyhydroxide respectively generated inthe reverse current absorption body by electrolysis proceed in thefollowing order, thereby absorbing the reverse current.

O₂+2H₂O+4e ⁻→4OH⁻  (5)

NiO₂+H₂O+e ⁻→NiOOH+OH⁻  (6)

NiOOH+H₂O+e ⁻→Ni(OH)₂+OH⁻  (7)

By employing the reverse current absorption body 234 of the presentreference embodiment, much of the reverse current can be consumed by thereactions (6) and (7). In the period that the reactions (5), (6) and (7)are occurring, the anode 214 having the same potential as the reversecurrent absorption body 234 does not undergo cathodic polarization. Evenin the case of a catalyst material that is stable during normalelectrolysis, if a large cathodic polarization occurs once, then whenelectrolysis is performed again, anodic polarization can cause elutionof the catalyst and a loss of conductivity. Accordingly, by housing thereverse current absorption body 234 of the present embodiment inside theanode chamber 210 of an alkaline water electrolytic apparatus, anodedegradation caused by reverse current can be prevented.

In this description, the electric capacity of the period up untilcompletion of the reactions (5), (6) and (7) is defined as the dischargecapacity on the anode side.

The second reference embodiment of the present invention was describedusing a bipolar alkaline water electrolytic apparatus as an example, butthe same effects can be obtained with a monopolar alkaline waterelectrolytic apparatus.

First Embodiment

A first embodiment of the present invention is described using a bipolarbrine electrolytic apparatus. The effects of this embodiment can also beobtained in a monopolar brine electrolytic apparatus or an alkalisulfate electrolytic apparatus.

FIG. 5 illustrates an electrolytic cell used in the brine electrolyticapparatus of the first embodiment, and illustrates a cross-sectionalview along A-A′ in FIG. 1 (a horizontal cross-sectional view).

With the exception of the installation location for the reverse currentabsorption body, the first embodiment has the same structure as thefirst embodiment. Accordingly, an anode 314 and a buffer plate 316disposed inside an anode chamber 310 are the same as those described inthe first reference embodiment.

In an electrolytic cell 300 in the first embodiment, a reverse currentabsorption body 334 of the type described in the first referenceembodiment is coupled to a cathode current collector 328 inside acathode chamber 320. The reverse current absorption body 334 may be abody that has been immersed in a solution containing an alkali metalhydroxide following sintering, thereby removing alkali-solublecomponents from regions near the surface. Alternatively, the sinteredcompact obtained following sintering may be used, as is, as the reversecurrent absorption body 334, and coupled to the cathode currentcollector 328.

The reverse current absorption body 334 may be installed on the surfaceof the cathode current collector 328 on the side facing the cathode 330,or may be installed on the surface on the side facing a partition wall304 (the surface on the opposite side from the cathode 330). The reversecurrent absorption body 334 is coupled to the cathode current collector328 in a detachable manner. The method used for coupling the reversecurrent absorption body 334 to the cathode current collector 328 mayemploy similar methods to those described in the first referenceembodiment such as securing by wire or welding. In those cases where thereverse current absorption body 334 is disposed on the cathode-sidesurface of the cathode current collector 328, the reverse currentabsorption body 334 can be secured by sandwiching between the cathodecurrent collector 328 and an elastic body 326. Further, the reversecurrent absorption body 334 may be coated with a wire gauze such as awoven mesh before being coupled to the cathode current collector 328.

As described above in the first reference embodiment, a conductivesubstrate 332 of the cathode 330, the elastic body 326, the cathodecurrent collector 328, a frame 302, and supports 308 are formed frommetal materials having conductivity. As a result, the cathode 330 andthe reverse current absorption body 334 are connected electrically.

When operation of the electrolyzer is stopped and a reverse current isgenerated, the reactions (1) and (2) proceed. Because the cathode 330 ismaintained at the same potential as the reverse current absorption body334, oxidation reactions at the cathode 330 (the reactions (3) and (4))do not proceed while the reactions (1) and (2) are occurring, meaningthe cathode catalyst is protected.

Second Embodiment

A second reference embodiment of the present invention is an alkalinewater electrolytic apparatus. FIG. 6 illustrates an electrolytic cellapplicable to the second embodiment, and illustrates a cross-sectionalview along A-A′ in FIG. 1 (a horizontal cross-sectional view). FIG. 6illustrates an example in which the embodiment is applied to a bipolaralkaline water electrolyzer, but this embodiment may also be applied toa monopolar electrolyzer.

With the exception of the installation location for the reverse currentabsorption body, the second embodiment has the same structure as thesecond reference embodiment.

In an electrolytic cell 400 of the second embodiment, a reverse currentabsorption body 434 of the type described in the first referenceembodiment is coupled to at least one of an anode current collector 418and a cathode current collector 428. The reverse current absorption body434 may be a body that has been immersed in advance in a solutioncontaining an alkali metal hydroxide (the production step A), or may bea body that has not been treated following sintering (the productionstep B).

As illustrated in FIG. 6, the anode current collector 418 and thecathode current collector 428 may each be located on either a surface onthe electrode side (an anode 414 or a cathode 430) or a surface on theside of a partition wall 404. The reverse current absorption bodies 434are coupled to the anode current collector 418 and the cathode currentcollector 428 in a detachable manner. The method used for performing thecoupling to the anode current collector 418 and the cathode currentcollector 428 may employ securing by wire, or welding or the like. Inthose cases where the reverse current absorption body 434 is disposed onthe electrode-side surface, the reverse current absorption body 434 maybe secured by sandwiching between the anode current collector 418 and anelastic body 416, or between the cathode current collector 428 and anelastic body 426. Further, the reverse current absorption bodies 434 maybe coated with a wire gauze such as a woven mesh before being coupled tothe anode current collector 418 and the cathode current collector 428.

A conductive substrate 432 of the cathode 430, the elastic body 426, thecathode current collector 428, a frame 402, and support members 408 areformed from metal materials having conductivity. As a result, thecathode 430 and the reverse current absorption body 434 are connectedelectrically inside a cathode chamber 420.

Further, the anode 414, the elastic body 416 and the anode currentcollector 428 are formed from metal materials having conductivity. As aresult, the anode 414 and the reverse current absorption body 434 areconnected electrically inside an anode chamber 410.

As mentioned above, the reverse current absorption bodies 434 areconnected electrically to the anode 414 or the cathode 430. Accordingly,when operation of the electrolyzer is stopped and a reverse current isgenerated, the reactions (1) and (2) proceed at the cathode side.Because the cathode 430 is maintained at the same potential as thereverse current absorption body 434, oxidation reactions do not proceedat the cathode 430 while the reactions (1) and (2) are occurring,meaning the catalyst is protected. Further, the reactions (5), (6) and(7) proceed at the anode side, and because the anode 414 is maintainedat the same potential as the reverse current absorption body 434,cathodic polarization does not occur beyond this potential. As a result,when electrolysis is restarted, anode degradation due to catalystelution or a deterioration in conductivity can be prevented.

Third Embodiment

A third embodiment of the present invention is a brine electrolyticapparatus. FIG. 7 illustrates an electrolytic cell applicable to thethird embodiment, and illustrates a cross-sectional view along A-A′ inFIG. 1 (a horizontal cross-sectional view). FIG. 8 also illustrates anelectrolytic cell applicable to the third embodiment, and illustrates across-sectional view along B-B′ in FIG. 1 (a vertical cross-sectionalview). FIG. 7 and FIG. 8 illustrate an example in which the embodimentis applied to a bipolar brine electrolyzer, but this embodiment may alsobe applied to a monopolar electrolyzer or an alkali sulfate electrolyticapparatus.

With the exception of the installation location for the reverse currentabsorption body, the third embodiment has the same structure as thefirst reference embodiment. Accordingly, an anode 514 and a buffer plate516 disposed inside an anode chamber 510 are the same as those describedin the first reference embodiment.

In an electrolytic cell 500 in the third embodiment, a reverse currentabsorption body 534 of the type described in the first referenceembodiment is coupled to an electrolytic cell structural member inside acathode chamber 520. The reverse current absorption body 534 may be abody that has been immersed in advance in a solution containing analkali metal hydroxide (the production step A), or may be a body thathas not been treated following sintering (the production step B).

In the third embodiment, the electrolytic cell structural member refersto a member that constitutes part of the electrolytic cell besides acathode structure (a cathode 530, a cathode current collector 528 and anelastic body 526) and an anode 514, and specific examples of this memberinclude a frame 502, a partition wall 504, a support member (rib) 508, agasket 506, and a buffer plate 516. However, in those cases where thesupport members 508 are integrated with the cathode current collector528 and the elastic body 526, this type of integrated structure is alsoincluded within the definition of electrolytic cell structural members.

Accordingly, in the third embodiment, reverse current absorption bodies534 are attached, in a detachable manner, to a surface of the partitionwall 504 on the side of the cathode chamber 520, a side wall 502A of theframe 502 on the inside of the cathode chamber, a surface of the frame502 on a bottom surface 502B (see FIG. 8) of the cathode chamber 520,and a support member 508. In the case of support members 508 that areintegrated with the cathode current collector and the elastic body, areverse current absorption body 534 may also be installed in a positionon the support member 508 on the side that contacts the cathode 530,provided it does not impair the function of the elastic body. Forexample, a spring-like structure or coil-like structure may be formed onthe support member 508 as an elastic body that contacts the cathode 530,and a reverse current absorption body 534 may be attached in a locationin which the elastic body is not formed.

The reverse current absorption bodies 534 are preferably coupled to theelectrolytic cell structural members described above by welding, but mayalso be attached by metal securing means such as wires, provided thebodies can be secured satisfactorily. Further, the reverse currentabsorption bodies 534 may be coated with a wire gauze such as a wovenmesh before being coupled to the electrolytic cell structural members.

The reverse current absorption bodies 534 and the cathode 530 areconnected electrically via the partition wall 504, the frame 502, thesupport members 508, the cathode current collector 528 and the elasticbody 526.

When operation of the electrolyzer is stopped and a reverse current isgenerated, the cathode 530 is maintained at the same potential as thereverse current absorption bodies 534. As a result, oxidation reactionsat the cathode 530 do not occur while the oxidation reactions of thereactions (1) and (2) are proceeding in the reverse current absorptionbodies 534, meaning the catalyst is protected.

Fourth Embodiment

A fourth embodiment of the present invention is an alkaline waterelectrolytic apparatus. FIG. 9 illustrates an electrolytic cellapplicable to the fourth embodiment, and illustrates a cross-sectionalview along A-A′ in FIG. 1 (a horizontal cross-sectional view). FIG. 9illustrates an example in which the embodiment is applied to a bipolaralkaline water electrolyzer, but this embodiment may also be applied toa monopolar electrolyzer.

With the exception of the installation location for the reverse currentabsorption bodies, the fourth embodiment has the same structure as thesecond reference embodiment. In an electrolytic cell 600 in the fourthembodiment, reverse current absorption bodies 634 of the type describedin the first reference embodiment are coupled to electrolytic cellstructural members inside an anode chamber 610 and a cathode chamber620. In the fourth embodiment, the electrolytic cell structural membersrefer to members that constitute part of the electrolytic cell besides acathode structure (a cathode 630, a cathode current collector 628 and anelastic body 626) and an anode structure (an anode 614, an anode currentcollector 618 and an elastic body 616), and specific examples of thesemembers include a frame 602, a partition wall 604, support members(ribs) 608, and a gasket 606. Accordingly, the reverse currentabsorption bodies 634 are attached to the partition wall 604, the innerwall surface of the frame 602, the bottom surface of the frame (notshown in FIG. 9), and the support members 608. The reverse currentabsorption bodies 634 may be installed in only one of the anode chamber610 and a cathode chamber 620, or may be installed in both chambers.

In this embodiment, the support members 608 may be integrated with thecathode current collector and the elastic body, or integrated with theanode current collector and the elastic body, and these types ofintegrated structures are also included within the definition ofelectrolytic cell structural members. In the case of this type ofintegrated structure, the reverse current absorption body 634 may beinstalled in a position on the side of the integrated structure thatcontacts the cathode 630 or the anode 614, in a similar manner to thatdescribed for the third embodiment.

The reverse current absorption bodies 634 are preferably coupled to theelectrolytic cell structural members described above by welding, but mayalso be attached by metal securing means such as wires. Further, thereverse current absorption bodies 634 may be coated with a wire gauzesuch as a woven mesh before being coupled to the electrolysis.

The reverse current absorption bodies 634 are connected electrically tothe cathode 630 and the anode 614 via the partition wall 604, the frame602, the support members 608, the cathode current collector 628, theanode current collector 618, and the elastic bodies 616 and 626.

When operation of the electrolyzer is stopped and a reverse current isgenerated, the reactions (1) and (2) proceed at the cathode side.Because the cathode 630 is maintained at the same potential as thereverse current absorption bodies 634, oxidation reactions do notproceed at the cathode 630 while the oxidation reactions (1) and (2) areoccurring, meaning the catalyst formed on the cathode 630 is protected.Further, the reactions (5), (6) and (7) proceed at the anode side, andbecause the anode 614 is maintained at the same potential as the reversecurrent absorption bodies 634, cathodic polarization does not occurbeyond this potential. As a result, when electrolysis is restarted,anode degradation due to catalyst elution or a deterioration inconductivity can be prevented.

EXAMPLES Reference Example 1

First, 0.5 g of a raw material powder obtained by mixing metallic nickelparticles (average particle size: 4.5 μm) and Raney nickel (Ni—Al)particles (Ni:Al=50:50 (mass ratio), average particle size: 45 μm) in aratio of 50:50 (mass ratio) was molded under the following conditions.

Molded body size: diameter 10 mm×thickness 1.4 mm

Molding pressure: 740 MPa

The molded body was sintered at 700° C. for 2 hours. The thus obtainedsintered compact was immersed in a 30 wt % aqueous solution of NaOH at90° C. for 2 hours to elute the alkali-soluble component (Al) in thesintered compact.

Using Ni wire, the above sintered compact was secured as a reversecurrent absorption body to an active cathode with an area of 4 cm². Anickel woven mesh having a catalyst layer containing Ru formed on thesurface was used as the active cathode. The sintered compact wasattached to the elastic body-side surface of the cathode insubstantially the central portion of the cathode.

An elastic body (coil cushion formed from nickel) and the above cathodewere disposed on a cathode current collector (pure nickel expandedmetal) to form a cathode structure. Using this cathode structure,electrolysis was performed under the following conditions.

Counter electrode: Ni expanded mesh

Electrolyte: 30 wt % aqueous solution of NaOH, temperature: 90° C.

Current density during electrolysis: 10 kA/m²

Electrolysis time: 1 hour

After completion of the electrolysis, a reverse current of 400 A/m² wasapplied. The discharge capacity of Reference Example 1 was calculatedfrom the amount of electricity required for the cathode potential toreach 0 V (vs. Hg/HgO). The discharge capacity varies depending on theexperimental conditions, but in this example, is the value obtained whenelectrolysis was performed under the above electrolysis conditions forone hour at 10 kA/m², and a reverse current of 400 A/m² was subsequentlyapplied.

Example 1

A sintered compact prepared using the same method as Reference Example 1was disposed as a reverse current absorption body on a cathode currentcollector (pure nickel expanded metal). A woven mesh (of nickel) wasmounted on top of the sintered compact, and an elastic body (coilcushion formed from nickel) and an active cathode with an area of 4 cm²(having a catalyst layer containing Ru formed on the surface of a nickelwoven mesh) were then disposed on top, forming a cathode structure.Using this cathode structure, electrolysis was performed under the sameconditions as Reference Example 1.

After completion of the electrolysis, a reverse current of 400 A/m² wasapplied. The discharge capacity of Reference Example 1 was calculatedfrom the amount of electricity required for the cathode potential toreach 0 V (vs. Hg/HgO).

Comparative Example 1

A substrate (nickel expanded metal) was subjected to Raney nickeldispersive plating, forming a reverse current absorption body providedwith a thin-film reverse current absorption layer of about 300 μm.

This reverse current absorption body of Comparative Example 1 was usedas a current collector, and combined with a similar elastic body andcathode to those used in Example 1 to prepare a cathode structure. Thiscathode structure was then subjected to electrolysis and reverse currentapplication under the same conditions as Reference Example 1, and thedischarge capacity of Comparative Example 1 was calculated.

Comparative Example 2

The cathode, elastic body and current collector described in Example 1were combined to prepare a cathode structure. Using this cathodestructure of Comparative Example 2, electrolysis and reverse currentapplication were performed under the same conditions as ReferenceExample 1, and the discharge capacity of Comparative Example 2 wascalculated.

The discharge capacity of Reference Example 1 was 13.99 mF/g (95.53F/m²). The discharge capacity of Example 1 was 14.18 mF/g (96.88 F/m²).In contrast, the discharge capacity of Comparative Example 1 was 3.31F/m², and the discharge capacity of Comparative Example 2 was 0.07 F/m².In this manner, it is evident that when a reverse current absorptionbody formed from a sintered compact is installed (Example 1, ReferenceExample 1), the discharge capacity improves dramatically.

Reference Example 2

Using the same cathode structure as Reference Example 1, electrolysiswas performed at a current density of 10 kA/m² for 12 hours.Subsequently, a cycle of applying a reverse current for 5 hours wasrepeated 100 times. The reverse current was applied so that thecumulative amount of electricity relative to the reverse currentabsorption body was 3.66 mF/g (25 F/m²) per cycle.

Comparative Example 3

Using the same cathode structure as Comparative Example 2, electrolysisand reverse current application were performed under the same conditionsas Reference Example 2.

FIG. 10 is a graph illustrating the change in the hydrogen overvoltageof the active cathode during the reverse current application cycles. Inthe figure, the horizontal axis represents the cycle number, and thevertical axis represents the hydrogen overvoltage when the currentdensity is 6 kA/m².

In Reference Example 2, no ruthenium elution was confirmed during the100 cycles. The increase in the hydrogen overvoltage followingcompletion of the 100 cycles was about 10 to 20 mV, indicating almost nodegradation.

In contrast, in Comparative Example 3, ruthenium elution was confirmedin each cycle. After the completion of 15 cycles, the hydrogenovervoltage of the active cathode had increased about 150 mV from theinitial value. In the case of Comparative Example 3, the test was haltedafter 15 cycles.

As described above, it was evident that by installing a reverse currentabsorption body of the present invention, the cathode performance couldbe maintained even after exposure to reverse current over a long period.

Example 3

Using 0.5 g of a raw material powder obtained by mixing metallic nickelparticles of different particle sizes and Raney nickel particles (thesame as Reference Example 1) in a ratio of 50:50 (mass ratio), a seriesof sintered compacts were produced and the alkali-soluble componentswere eluted under the following conditions.

Molded body size: diameter 10 mm×thickness 1.4 to 1.5 mm

Molding pressure: 740 MPa

Sintering temperature: 700° C.

Sintering time: 2 hours

Immersion solution: 30 wt % aqueous solution of NaOH, 90° C.

Immersion time: 7 hours

Using the same procedure as Reference Example 1, the thus obtainedsintered compacts were each coupled as a reverse current absorption bodyto an active cathode (of the same type as Reference Example 1), thusforming a series of cathode structures. Electrolysis and reverse currentapplication were then performed under the same conditions as Example 1,and the discharge capacity of each example was calculated. The resultsare shown in Table 1.

TABLE 1 Metallic nickel particles average Discharge capacity particlesize (μm) (mF/g) (F/m²) Sample 1 0.13 2.24 15.39 Sample 2 0.2 3.13 20.16Sample 3 0.4 5.83 39.58 Sample 4 2.5 6.38 44.34 Sample 5 4 9.23 62.65Sample 6 10 2.92 20.02 Sample 7 50 2.06 13.27 Sample 8 100 The sampledid not maintain its shape after immersion

The results revealed that a high discharge capacity of at least 1.5 mF/g(at least 10 F/m²) was obtained across the entire average particle sizerange from 0.13 to 50 μm (samples 1 to 7). When the average particlesize of the metallic nickel particles was from 0.2 to 10 μm (samples 2to 6), a very high discharge capacity exceeding 2.96 mF/g (20 F/m²) wasobtained. Further, an even higher discharge capacity was obtained whenthe average particle size of the metallic nickel particles was within arange from 0.4 to 4 μm, and particularly when the average particle sizewas from 2.5 to 4 μm.

Example 4

Ni—Al—Ru—Sn Raney alloy particles (average particle size: 45 μm) wereprepared as Raney nickel alloy particles. The composition of the alloyparticles was Ni:Al:Ru:Sn=35.6:49.4:1:14 (mass ratio). These Raneynickel alloy particles and the metallic nickel particles of ReferenceExample 1 were mixed together in a ratio of 50:50 (mass ratio) to obtaina raw material powder.

Example 5

Ni—Al—Ti—Ru—Co Raney alloy particles (average particle size: 45 μm) wereprepared as Raney nickel alloy particles. The composition of the alloyparticles was Ni:Al:Ti:Ru:Co=50.2:45.8:2:1:1 (mass ratio). These Raneynickel alloy particles and the metallic nickel particles of ReferenceExample 1 were mixed together in a ratio of 50:50 (mass ratio) to obtaina raw material powder.

Using 0.5 g samples of the raw material powders of Examples 4 and 5,molding was performed under the following conditions.

Molded body size: diameter 10 mm×thickness 1.4 mm

Molding pressure: 740 MPa

Each molded body was sintered at 700° C. for 2 hours. The thus obtainedsintered compacts were each immersed in a 30 wt % aqueous solution ofNaOH at 90° C. for 7 hours to elute the alkali-soluble component (Al) inthe sintered compact.

Using the same procedure as Reference Example 1, each of the abovesintered compacts was coupled as a reverse current absorption body to anactive cathode (of the same type as Reference Example 1), thus forming acathode structure. Electrolysis and reverse current application werethen performed under the same conditions as Reference Example 1, and thedischarge capacity was calculated.

The discharge capacity of Example 4 was 10.84 mF/g (73.66 F/m²). Thedischarge capacity of Example 5 was 3.60 mF/g (24.44 F/m²). Theseexperiments showed that a high discharge capacity could be obtained evenin cases where a multicomponent Raney nickel alloy was used.

Example 6

Using 0.5 g samples of raw material powders obtained by mixing metallicnickel particles (average particle size: 4 μm) and Raney nickel (Ni—Al)particles (Ni:Al=50:50 or 40:60 (mass ratio), average particle size: 45μm) in various mixing ratios, sintered compact samples 9 to 22 havingvarious nickel content and density values were prepared. Molding andelution of the alkali-soluble component were performed under thefollowing conditions.

Molded body size: diameter 10 mm×thickness 0.9 to 2.1 mm

Molding pressure: 740 MPa

Sintering temperature: 700° C.

Sintering time: 2 hours

Immersion solution: 30 wt % aqueous solution of NaOH, 90° C.

Immersion time: 24 hours

Using the same procedure as Reference Example 1, the thus obtainedsintered compacts were each coupled as a reverse current absorption bodyto an active cathode (of the same type as Reference Example 1), thusforming a series of cathode structures. Electrolysis and reverse currentapplication were then performed under the same conditions as ReferenceExample 1, and the discharge capacity of each example was calculated.The results are shown in Table 2. In Table 2, the “nickel content”represents the value determined from the total amount of nickel in themetallic nickel particles and the nickel-aluminum alloy particles priorto the immersion step.

TABLE 2 Amount of Density metallic nickel Ni/Al ratio Nickel beforeafter Change in Discharge Sample particles of Raney content immersionimmersion surface layer after capacity name (% by mass) nickel alloy (%by mass) step (g/cm³) step (g/cm³) immersion step (mF/g) (F/m²) Sample 90 40/60 40.0 2.91 — sample shape — — was not maintained Sample 10 540/60 43.0 2.93 1.27 sample shape — — was not maintained Sample 11 1540/60 49.0 3.26 2.18 some loss of 10.56 72.5 surface layer particlesSample 12 5 50/50 52.5 3.44 2.09 some loss of 9.45 64.6 surface layerparticles Sample 13 24 40/60 54.4 3.50 2.59 no change 12.91 85.8 Sample14 15 50/50 57.5 3.63 2.87 no change 9.03 60.2 Sample 15 25 50/50 62.53.90 3.39 no change 7.52 50.5 Sample 16 50 50/50 75.0 4.92 4.65 nochange 9.23 62.7 Sample 17 65 50/50 82.5 5.56 5.18 no change 4.07 27.3Sample 18 75 50/50 87.5 5.95 5.62 no change 2.98 20.1 Sample 19 80 50/5090.0 6.51 6.38 no change 1.55 10.2 Sample 20 85 50/50 92.5 6.94 6.89 nochange 0.85 5.5 Sample 21 90 50/50 95.0 7.35 7.31 no change 0.38 2.5Sample 22 100 50/50 100.0 7.58 7.58 no change 0.06 0.4

In samples 9 and 10, which either contained no metallic nickelparticles, or contained metallic nickel particles but had a low totalnickel content, the hydrogen gas generated from the sample during theimmersion step meant that the sample itself was unable to maintain itsshape, and the sample disintegrated. Further, in samples 11 and 12 inwhich the nickel content was low, although the loss of some surfaceparticles was observed during the immersion step, the sample shape wasmaintained.

The relationship between the discharge capacity and the sintered compactdensity in Table 2 is shown in FIG. 11. The values before the immersionstep have a curve (solid line in the figure) that decreases gentlytoward the right from the value when the nickel content is 100% (7.58g/cm³, 0.06 mF/g). Further, the values after the immersion step alsodecrease toward the right from the value when the nickel content is 100%(7.58 g/cm³, 0.06 mF/g), and follow a curve (dashed line in the figure)positioned below the curve for the values before the immersion step.

Example 7

By mixing metallic nickel particles (average particle size: 50 μm) andRaney nickel (Ni—Al) particles (Ni:Al=40:60 (mass ratio), averageparticle size: 45 μm), and using a 0.5 g sample of a raw material powderhaving a different mixing ratio from sample 7 of Example 3, a sinteredcompact 22 having different nickel content and density values wasprepared. Molding and elution of the alkali-soluble component wereperformed under the following conditions.

Molded body size: diameter 10 mm×thickness 1.9 mm

Molding pressure: 740 MPa

Sintering temperature: 700° C.

Sintering time: 2 hours

Immersion solution: 30 wt % aqueous solution of NaOH, 90° C.

Immersion time: 24 hours

Using the same procedure as Reference Example 1, the thus obtainedsintered compacts were each coupled as a reverse current absorption bodyto an active cathode (of the same type as Reference Example 1), thusforming a series of cathode structures. Electrolysis and reverse currentapplication were then performed under the same conditions as ReferenceExample 1, and the discharge capacity of each example was calculated.The results are shown in Table 3.

TABLE 3 Amount of Density metallic nickel Ni/Al ratio Nickel beforeafter Change in Discharge Sample particles of Raney content immersionimmersion surface layer after capacity name (% by mass) nickel alloy (%by mass) step (g/cm³) step (g/cm³) immersion step (mF/g) (F/m²) Sample 550 50/50 75.0 4.39 3.92 no change 2.06 13.3 Sample 23 24 40/60 54.4 3.362.10 some loss of 4.64 30.2 surface layer particles

A graph obtained by adding the relationship between the dischargecapacity and the sintered compact density in Table 3 to the values inFIG. 11 is shown in FIG. 12. When the average particle size of themetallic nickel particles is 50 μm, the values exhibit a similardischarge capacity—sintered compact density relationship to thatobserved when the average particle size is 4 μm. In other words, thevalues before the immersion step have a curve (solid line in the figure)that decreases gently toward the right from the value when the nickelcontent is 100% (7.58 g/cm³, 0.06 mF/g). Further, the values after theimmersion step also decrease gently toward the right from the value whenthe nickel content is 100% (7.58 g/cm³, 0.06 mF/g), and follow a curve(dashed line in the figure) positioned below the curve for the valuesbefore the immersion step. Based on the results of Example 6 and Example7, it is evident that this discharge capacity—sintered compact densityrelationship applies even when the particle size of the metallic nickelparticles is changed.

Moreover, the results obtained by adding the measurement results for thesintered compact density after the immersion step for the samples 1 to 7from Example 3 are shown in Table 4. Further, a graph obtained by addingthe relationship between the discharge capacity and the sintered compactdensity in Table 4 to the values in FIG. 12 is shown in FIG. 13.

TABLE 4 Amount of Density metallic nickel Ni/Al ratio Nickel beforeafter Change in Discharge Sample particles of Raney content immersionimmersion surface layer after capacity name (% by mass) nickel alloy (%by mass) step (g/cm³) step (g/cm³) immersion step (mF/g) (F/m²) Sample 150 50/50 75.0 4.63 4.22 no change 2.24 15.4 Sample 2 50 50/50 75.0 4.534.21 no change 3.13 21.2 Sample 3 50 50/50 75.0 4.89 4.53 no change 5.8339.6 Sample 4 50 50/50 75.0 4.82 4.48 no change 6.38 44.3 Sample 5 5050/50 75.0 4.92 4.65 no change 9.23 62.7 Sample 6 50 50/50 75.0 4.554.22 no change 2.92 20.0 Sample 7 50 50/50 75.0 4.39 3.92 no change 2.0613.3

By changing the average particle size of the nickel, the relationshipbetween the discharge capacity and the sintered compact density changesin the manner shown in the shaded region in FIG. 13, and satisfies thecorrelations shown in FIG. 11 and FIG. 12 at each particle size. FromFIG. 13 it is evident that the results at which the dischargecapacity—sintered compact density correlation is farthest right are whenthe average particle size of the nickel is 4 μm. If this observation isconsidered in combination with the fact that the dischargecapacity—sintered compact density correlation is a curve that decreasesgently toward the right, then the sintered compact densities thatexhibit the same amount of reverse current absorption exhibit a maximumwhen the nickel average particle size is 4 μm.

In order to achieve the effects of the present invention, the sinteredcompact must have a discharge capacity of at least 1.5 mF/g (10 F/m²).In terms of satisfying this condition, the upper limit for the densityof a sintered compact of nickel particles having an average particlesize of 4 μm is 6.51 g/cm³.

Based on the above Example 6 and Example 7, the following facts areevident. The nickel content in the reverse current absorption body ispreferably from 45 to 90% by mass. Provided the nickel content is withinthis range, the shape of the reverse current absorption body can bemaintained even after the immersion step, and a high discharge capacityexceeding 1.5 mF/g (10 F/m²) can be obtained. From the viewpoint ofdurability, this nickel content is more preferably from 53 to 90% bymass. Moreover, if the reverse current absorption performance is alsotaken into consideration, then the nickel content is even morepreferably from 53 to 87.5% by mass, and is most preferably from 53 to77.5% by mass.

Further, the density of the reverse current absorption body ispreferably from 2.00 to 6.51 g/cm³. Provided the density is within thisrange, the shape of the reverse current absorption body can bemaintained even after the immersion step, and a high discharge capacityexceeding 1.5 mF/g (10 F/m²) can be obtained. From the viewpoint ofdurability, this nickel content is more preferably from 2.30 to 6.51g/cm³. Moreover, if the reverse current absorption performance is alsotaken into consideration, then the density is even more preferably from2.30 to 5.95 g/cm³, and is most preferably from 2.30 to 5.10 g/cm³.

DESCRIPTION OF THE REFERENCE SIGNS

-   10, 100, 140, 200, 300, 400, 500, 600: Electrolytic cell-   12: Electrode-   14, 106, 206, 506, 606: Gasket-   102, 202, 302, 402, 502, 602: Frame-   104, 204, 304, 404, 504, 604: Partition wall-   108, 208, 308, 408, 508, 608: Support member-   110, 210, 310, 410, 510, 610: Anode chamber-   114, 214, 314, 414, 514, 614: Anode-   116, 316, 516: Buffer plate-   120, 220, 320, 420, 520, 620: Cathode chamber-   122, 222: Cathode structure-   126, 216, 226, 326, 416, 426, 526, 616, 626: Elastic body-   128, 228, 328, 428, 528, 628: Cathode current collector-   130, 230, 330, 430, 530, 630: Cathode-   132, 232, 332, 432: Conductive substrate-   134, 234, 334, 434, 534, 634: Reverse current absorption body-   212: Anode structure-   218, 418, 618: Anode current collector

1. An electrolyzer comprising: an anode, an anode chamber housing theanode, a cathode, a cathode chamber housing the cathode, and a diaphragmthat separates the anode chamber and the cathode chamber, wherein atleast one reverse current absorption body formed of a sintered compactcontaining nickel is disposed in at least one of an inside of thecathode chamber and an inside of the anode chamber, and the at least onereverse current absorption body is not directly coupled to the cathodeand the anode but is electrically connected to at least one of thecathode and the anode.
 2. The electrolyzer according to claim 1, furthercomprising a cathode current collector disposed in the cathode chamberand opposed to the cathode, wherein the at least one reverse currentabsorption body is coupled to the cathode current collector.
 3. Theelectrolyzer according to claim 1, further comprising an anode currentcollector disposed in the anode chamber and opposed to the anode,wherein the at least one reverse current absorption body is coupled tothe anode current collector.
 4. The electrolyzer according to claim 1,wherein the at least one reverse current absorption body is attached toat least one of a frame body defining the cathode chamber and a firstsupport member disposed in the cathode chamber and supporting thecathode.
 5. The electrolyzer according to claim 1, wherein the at leastone reverse current absorption body is attached to at least one of aframe body defining the anode chamber and a second support memberdisposed in the anode chamber and supporting the anode.
 6. Theelectrolyzer according to claim 1, wherein the content of nickel in thesintered compact containing nickel is 45 to 90 mass %.
 7. Theelectrolyzer according to claim 1, wherein a density of the at least onereverse current absorption body is 2.00 to 6.51 g/cm³.